1. Field of the Invention
This invention relates to the field of radio frequency antennas. More particularly to the use of a radio frequency antenna as a transseptal needle for use in vivo in conjunction with magnetic resonance imaging techniques.
2. Description of Related Art
Magnetic resonance imaging (MRI) is a well known, highly useful technique for imaging matter. It has particular use with imaging the human body or other biological tissue without invasive procedures or exposure to the harmful radiation or chemicals present with x-rays or CT scans. MRI uses changes in the angular momentum or “spin” of atomic nuclei of certain elements to show locations of those elements within matter. In an MRI procedure, a subject is usually inserted into an imaging machine that contains a large static magnetic field generally on the order of 0.2 to 4 Tesla although machines with higher strength fields are being developed and used. This static magnetic field tends to cause the vector of the magnetization of the atomic nuclei placed therein to align with the magnetic field. The subject is then exposed to pulses of radio frequency (RF) energy in the form of a second, oscillating, RF magnetic field having a particular frequency referred to in the art as a resonant or Larmor frequency. This frequency is equal to the rate that the spins rotate or precess.
This second field is generally oriented so that its magnetic field is oriented in the transverse plane to that of the static magnetic field and is generally significantly smaller. The second field pulls the net magnetism of the atomic nuclei off the axis of the original magnetic field. As the second magnetic field pulses, it pulls the spins off axis. When it is turned off, the spins “relax” back to their position relative to the initial magnetic field. The rate at which the spins relax is dependent on the molecular level environment. During the relaxation step, the precessing magnetization at the Larmor frequency induces a signal voltage that can be detected by antennas tuned to that frequency. The magnetic resonance signal persists for the time it takes for the spin to relax. Since different tissues have different molecular level environments, the differences in relaxation times provides a mechanism for tissue contrast in MRI.
In order to image the magnetic resonance signal it is necessary to encode the locations of the resonant spins. This is performed by applying pulse of gradient magnetic fields to the main magnetic field in each of the three dimensions. By creating this field, the location of resonant nuclei can be determined because the nuclei will resonate at a different Larmor frequency since the magnetic field they experience differs from their neighbors. The magnetic resonance (MR) image is a representation of the magnetic resonance signal on a display in two or three dimensions. This display usually comprises slices taken on an axis of interest in the subject, or slices in any dimension or combination of dimensions, three-dimensional renderings including computer generated three-dimensional “blow-ups” of two-dimensional slices, or any combination of the previous, but can comprise any display known to the art.
MR signals are very weak and therefore the antenna's ability to detect them depends on both its size and its proximity to the source of those signals. In order to improve the signal of an MRI, the antenna may be placed near or inside the subject to be imaged. Such improvements can enable valuable increases in resolution sensitivity and reduction of scan time. It may be desirable to have evidence of the MRI antenna itself on the MRI to allow the individual inserting the MRI antenna to direct where it is going and to maneuver it with aid from the MR image. Such a benefit could be useful in medical procedures where MRI is used simultaneously to track the position of an intraluminal device and to evaluate the structures surrounding the lumen. For example, an intravascular catheter could be directed through a vessel using MRI to reach a targeted area of the vessel, and the MRI apparatus could further be used to delineate the intravascular anatomy or nearby tissue to determine whether a particular therapeutic intervention would be required. Using MRI to guide the catheter and using MRI further to map out the relevant anatomy and perform an intervention could complement conventional angiographic imaging technology within an interventional radiology or cardiology or minimally invasive imaging suite. Once the catheter is directed to the desired anatomic target under MR guidance, and once the topography or other relevant anatomy of the target lesion is depicted using MRI, the clinician can make decisions about what type of intervention would be indicated, if any, and where the intervention should be delivered.
Many conventional vascular interventional procedures use X-ray imaging technology in which transseptal needles and catheters are inserted into a vein or artery and navigated to specific locations in the heart for diagnostic and therapeutic procedures. Conventional X-ray guided vascular interventions, however, suffer from a number of limitations, including: (1) limited anatomical visualization of the body and blood vessels during the examination, (2) limited ability to obtain a cross-sectional view of the target vessel, (3) inability to characterize important pathologic features of atherosclerotic plaques, (4) limited ability to obtain functional information on the state of the related organ, and (5) exposure of the subject to potentially damaging x-ray radiation.
MRI techniques offer the potential to overcome these deficiencies. However, conventional transseptal needles are not suitable for use in MRI machines since they contain steel or magnetic materials that can cause significant image artifacts in an MRI machine and can cause injury to a patient from unintended motion due to effects of the magnetic fields or induced Ohmic heating. Even those antennae which have been fabricated for use inside a human body are not useful for many types of interventional procedures. Many of these devices are simply too large to be sufficiently miniaturized to fit in clinically used intravascular sheaths. Additionally, in order to be useful for procedures requiring loading of multiple tools during the procedure, it is desirable that the needle antenna be capable of loading multiple different tools after it has been placed in the subject.
X-ray fluoroscopy guided needle puncture of the atrial septum through the fossa ovalis was initially independently described in 1959 and modified by Brockenbrough and Braunwald one year later. This approach quickly became the preferred means of catheter access to the left heart and experienced widespread use for a number of diagnostic and therapeutic applications including assessment of mitral valve disease and creation of atrial septal defects in children with congenital heart disease. Although used widely, transseptal left heart catheterization was recognized to be extremely time consuming, technically demanding and associated with high number of potentially life threatening risks. These early experiences, along with the development of increasingly sophisticated interventional and noninvasive techniques for accessing left heart hemodynamics, diminished the impetus for transseptal catheterization and the procedure fell into relative disuse by the late 1970's.
More recently, transseptal catheterization has experienced a revival due to the development of percutaneous transvenous balloon mitral valvuloplasty and curative left atrial radiofrequency catheter ablation procedures. Fluoroscopy guided transseptal catheterization remains a technically difficult procedure, particularly in the setting of conditions that distort the normal atrial anatomy and the fluoroscopic position of the interatrial septum (e.g., atrial dilatation as is frequently encountered with mitral and/or tricuspid valve disease, aortic root dilatation, and hypertrophy of the interatrial septum). Associated complications of transseptal catheterization such as aortic, pulmonary artery and free wall atrial puncture can be serious and life threatening. Improvements in the technique and apparatus have yielded lower complication rates but even when performed by experienced operators, improper positioning of the device can result in cardiac or aortic perforation. Overall complication rates have been reported at 3 to 6%. ( ). With the decline in the use of transseptal techniques in diagnostic cardiology for assessment of valvular heart disease, there is likely to be a decreasing pool of experienced operators. Thus, increasing emphasis has been placed on vigilant, closely supervised training with emphasis placed upon properly identifying anatomic and catheter landmarks in the right atrium.
Over 3,000 transseptal procedures are performed each year in the United States for left sided radiofrequency ablation therapy and mitral valvoplasty procedures. Approximately 3-6% of these are complicated by aortic or atrial perforations due to incorrect needle positioning at the fossa ovalis. This relatively high complication rate can in part be attributed to the inability to directly visualize the fossa ovalis and other critical endocardial landmarks using a two-dimensional projection x-ray fluoroscopy.
MRI has several distinct advantages over x-ray fluoroscopy including, excellent soft-tissue contrast, the ability to define any tomographic plane and the absence of ionizing radiation exposure. In addition to these well-known general advantages, MRI offers several specific advantages that make it especially well suited for guiding transseptal puncture procedures including: 1) real-time interactive imaging, 2) direct visualization of critical endocardial anatomic landmarks, 3) direct high resolution imaging of the fossa ovalis, 4) visualization of the needle tip-tissue interface, 5) the ability to actively track needle position in three-dimensional space, and 6) elimination of radiation exposure. Despite this promise, MRI guided transseptal puncture has not been previously described. This can be most likely attributed to the many of the inherent challenges of therapeutic interventional cardiac MRI including device artifact, electromagnetic interference and the necessity for cardiac and respiratory gating and rapid imaging and display. The invention described herein describes a procedure and required hardware to perform MR guided transseptal puncture procedures with active needle tip tracking. Such a procedure and may have application in a number of interventional MRI guided therapies.
Previous studies demonstrated successful transseptal catheterization using transthoracic, transesophageal or intracardiac echocardiography. Each of these approaches, however, has significant practical and technical limitations. Transthoracic echocardiography lacks the resolution to accurately display the thin transseptal needle tip and single-plane images may provide misleading information regarding the position of the needle in three-dimensional space. In addition, x-ray fluoroscopy is required during imaging which may be logistically difficult, exposes the sonographer to radiation and may block fluoroscopic imaging. Transesophageal imaging with multi-plane probes improves spatial resolution considerably and thus overcomes some of these difficulties. While the needle tip can not be visualized, distention of the fossa immediately prior to perforation can be imaged. Transesophageal echocardiography guided procedures, however, are complicated by esophageal perforation or aspiration and the sedation required during prolonged esophageal intubation carries a risk of hypoventilation and limits communication with the patient during the procedure. Intracardiac echo guided has shown significant promise for guiding transseptal puncture procedures. Direct visualization of the fossa ovalis is possible and anatomic landmarks can be identified. Problems with this approach include, only limited views of the left and right atrium due to significant attenuation of sound at to high frequencies, the inability to distinguish multiple intracardiac catheters and the inability to track the needle tip and visualize fossa puncture. For these reasons, an alternative procedure would be desirable. MRI is not subject to these limitations and may be an ideal modality to guide transseptal puncture procedures. Recent published work has demonstrated that a complete electrophysiologic study can now be performed entirely under MRI guidance, including the ability to navigate catheters and characterize the temporal and spatial formation of ventricular radiofrequency ablation lesions in vivo. Additionally, MRI has been shown to guide mitral valvoplasty procedures. As these two therapies account for 95% of all transseptal procedures performed, it is clear that the ability to perform safe transseptal needle puncture under MRI guidance will be of great importance as the field of interventional cardiovascular MRI continues to evolve.